Epitaxial crystal growth means the deposition of a thin single-crystal film upon a substrate having a matching lattice structure. In the field of semiconductors, such crystal growth is performed most commonly on an oriented melt-grown single-crystal surface of the same material. Certain semiconductor devices, e.g., bipolar transistor ones, require epitaxial films in contrast to MOS transistor which do not require epitaxial films. Elemental germanium or silicon are grown in epitaxial films.
The conventional technique for growing epitaxial films is by chemical vapor deposition. Such processes are performed at relatively high operating tempertures, as on the order of 1200.degree.C. At such temperatures, the epitaxial growth of the layer over an underlying p-n junction tends to blur or smear the dopants by cross-diffusion. Thus, it is difficult to fabricate sharp junctions in thin intermediate layers at such temperatures. Another problem with said vapor deposition techniques is that they employ hazardous gases such as corrosive silicon tetrachloride and highly inflammable and explosive hydrogen and silicon tetrahydride. Also, by such techniques, film thickness is limited to a minimum of about one micron.
Silicon has been electrodeposited onto metallic substrates to form a silicide coating onto the metal substrate comprising an alloy of silicon and metal. One such system is set forth in U.S. Re. Pat. No. 25,630. However, there is no suggestion that the technique of electrodepositing silicon from molten salts could be employed to form an epitaxial crystal layer onto a substrate.